Microsoft Exchange 2007 OWA Mail Count does not tally with iPad Exchange

I discovered that when we first setup the iPad to sync email from Microsoft Exchange 2007 server via ActiveSync, the total number of unread email count tally with the count from OWA. After a few days, the unread email count will be out of sync. The settings for iPad "Mail days to sync": No limit. Mail to show: 1000. iPad iOS: 5.1.1

We performed the following:

1.     Compare email by email to verify which are the read email from OWA doesn't sync to iPad. We discovered that some emails read from OWA doesn't sync with iPad even performed forced update. However, if we delete that read email from OWA, it will delete (unread) from iPad immediately after sync.

2.     Email recall message for successful recalled displayed in OWA but not on iPad.

3.     Calendar invite message forward as email displayed in OWA but not in iPad.

Does anyone else face this issue too and any kind advice?

Thank you!